In a motor vehicle, it is known to use presence detection devices for detecting the presence of a hand or of a foot of a user of the vehicle and thus to allow all or some of the opening elements of the vehicle, for example the doors or the trunk, to be locked or unlocked. By way of example, the detection of the presence of a hand of a user on or in front of a door handle in conjunction with the recognition of an identifier of a “hands-free” access device carried by this user allows these opening elements to be locked and unlocked.
To achieve this, when the user approaches the vehicle, communication is established over a wireless communication link between the access device, for example an electronic fob or a mobile telephone, and the detection device in order to authenticate said detection device by virtue of its identifier.
To this end, the detection device includes an antenna allowing the identifier sent by the access device to be received. The detection device is connected to an electronic computer of the vehicle (ECU: abbreviation for “electronic control unit”) to which it transmits the identifier.
According to the prior art, the access device is generally an electronic fob. The signal received by the antenna of the detection device, comprising the identifier of the access device, is transmitted via RF (radiofrequency) or LF (low-frequency) waves.
Nowadays, however, it is increasingly common to use a mobile telephone to perform authentication functions, which makes it possible to avoid using a dedicated electronic fob and thus to limit the number of devices. Since the majority of mobile telephones do not have RF or LF communication means, it is known to use the Bluetooth® communication module, with which the majority of existing telephones are equipped, to send the identifier of the device in the case of a function of unlocking a vehicle.
Once the identifier has been received by the computer, said computer allows the access device to be recognized for the purpose of unlocking the vehicle only when the access device is situated close to the vehicle, for example less than one meter away.
It is known to determine this geographical proximity by determining the position of the access device in relation to the vehicle through triangulation. In one existing solution, the vehicle comprises three or four Bluetooth® antennas for receiving the signal, transmitted by access device, the signal strength of the received signal varying depending on the distance between the access device and each Bluetooth® antenna. The reception of the signal by all of the Bluetooth® antennas thus makes it possible, through triangulation, to determine the position of the access device in relation to the vehicle.
When the access device is located in what is known as a “proximity” zone by the computer, said computer allows the opening elements to be unlocked only when the carrier of the authorized access device places his hand on or close to a detection device installed in the vehicle, for example in a door handle.
To detect the presence of the hand of the user and to allow the opening elements of the vehicle to be unlocked, such a detection device comprises, in a known manner, a capacitive sensor.
According to one example of capacitive measurement, such a capacitive sensor comprises a first capacitor that is charged and discharged periodically from/into a second capacitor. When the first capacitor discharges into the second capacitor, the charges are balanced between the two capacitors.
When a hand is present on the handle or close to the handle, for example less than 10 mm away, the level of charge of the first capacitor increases. This results in a larger discharge of the first capacitor into the second capacitor, and therefore a higher level of balancing in the presence than in the absence of a hand on the handle. Such a sensor thus makes it possible to detect the intention of a user to unlock the opening elements of the vehicle.
However, it turns out that the presence of a hand on or close to the detection device influences the Bluetooth® antenna of said detection device. Specifically, in such a case, the hand of the user absorbs some of the waves exchanged between the detection device and the access device. The Bluetooth® antenna of the detection device then receives, from the access device, a signal whose measured signal strength value is reduced by the presence of the hand. The use of this incorrect value by the computer may lead to an error in the triangulation calculations. In particular, the access device may be detected in a position that is too far from the vehicle, for which the computer does not allow the opening elements to be unlocked even though it should, this constituting a major drawback.